Knowledge of the mechanisms employed by animal cells in regulating the production of cytoplasmic messenger RNA (mRNA) is fundamental for an understanding of development and how the factors which control normal cellular growth are altered in the cancerous state. It is now established that at least some cellular mRNA as well as mRNAs of the DNA tumor viruses are processed from a larger nuclear precursor RNA. We have been able to separate HeLa cell mRNAs into four classes of molecules: a) those containing Poly(A) sequences (Poly(A plus)) as well as Oligo(U) sequences (Oligo(U plus)) (4-13%); b) Poly(A plus) Oligo(U minus) (52%); c) Poly(A minus) Oligo(U plus) (2%); d) Poly(A minus) Oligo(U minus) (33%). These classes of mRNAs have been characterized with regard to size. It appears that the Oligo(U) is in an intramolecular duplex with the 3' Poly(A). The location of the Oligo(U) in the mRNAs will be determined with particular interest in whether the Poly(A plus) Oligo(U plus) mRNAs are in a circular configuration that may be under distinct translational control. The different mRNA classes will be translated in vitro to allow further identification. Also it will be determined whether the drug DRB effects the transcription or processing of hemoglobin mRNA in Friend cells. An understanding of these processes may reveal whether they are modified to alter the supply of mRNA according to growth or development requirements or changed when cells are transformed by DNA tumor viruses. Transformation is known to involve the integration of viral DNA into the host genome and the production of viral mRNA. It is not known how the presence of the viral mRNA(s) causes a loss of control of cellular growth, however, recent data indicate that the production of at least one of the host mRNAs coding for a large membrane protein is reduced.